I'm trying to figure out how to translate the 'on' when using days of the week (ie. on Thursday/s), but Iv'e seen different words used. Sometimes it's 'na' and sometimes 'as'... Normalmente vamos ao cinema às quintas-ferias. Tenho auka de aerobica na quinta. Is it just because one is singular and one is plural? But isn't the plural of 'na' 'nas'? Or is there some other reason for using different prepositions? Could we also say 'à quinta-feria' or 'nas quintas-ferias'?